The present invention relates to devices and methods for evaluating neurological function controlling muscular movements and, more particularly, for evaluating peripheral nerve function.
A peripheral motor nerve is composed of many nerve fibers which transmit or conduct electrical signals from the central nervous system to the muscles they control. Each nerve fiber branches at its termination and contacts as many as 200-300 muscle fibers, so that when a signal arrives at the terminals of a nerve fiber the muscle fibers also become electrically active. Muscle fibers are unusual and are distinguished from nerve fibers, however, in that once they are electrically active they also contract and generate force to move part of the body. It is common knowledge that very complicated sequential movements of one or more body parts can be produced (such as playing the piano) if the central nervous system can recruit the proper combination of nerve fibers for each movement in the sequence.
In many instances central nervous system command signals are generated but their conduction down nerve fibers is disturbed en route because of a pathological change or lesion in the peripheral nerve. In the initial stage of a lesion, signal conduction becomes less efficient so that there is a slowing of conduction in the nerve fibers affected. If the lesion intensifies, signal conduction may become blocked in these nerve fibers. Their corresponding muscle fibers can no longer be made to contract and the patient has a partial paralysis termed "a paresis". A lesion can intensify to block the entire nerve thereby producing a total paralysis.
In cases where a peripheral nerve lesion is suspected, it is important for the physician to test the nerve to determine if and to what extent signal conduction has been compromised. This information is not only important in predicting the outcome in a patient's paralysis (i.e. whether the patient will eventually recover from the paralysis or not) but also in deciding whether a particular form of treatment, such as surgery, should be advised. The most common method currently available to assess nerve signal conduction is the "nerve conduction velocity" test. Pairs of stimulus and recording electrodes are placed on the skin overlying the nerve on each side of the nerve lesion. When a stimulus is applied by passing electricity through the skin, nerve fibers are activated and a volley of signals (termed a "compound action potential") is conducted down the nerve through the lesion towards the recording electrodes. When the action potential reaches the recording electrodes, the voltage change detected by the electrodes can be amplified and displayed on an oscilloscope. The potential actually represents a compound voltage of all nerve fiber signals that made it through the lesion. The physician notes on the oscilloscope the time required for the potential to travel from the stimulus electrodes to the recording electrodes. By measuring the distance between the electrodes, he can calculate the average conduction velocity of the potential and compare it to normal values to gain some appreciation as to whether the lesion caused a slowing of signal conduction. The physician might also note that the recorded potential is subnormal in size suggesting that some nerve fiber signals were blocked at the lesion site. Unfortunately, the amplitude of a potential that can be recorded from a nerve through the skin is very small (approximately ten microvolts), so that decreases stemming from signal blocking are difficult to detect. This problem is compounded by the fact that slight changes in the recording electrode position can also cause significant changes in the size of the recorded potential. Thus, the method lacks adequate sensitivity and accuracy for assessing signal blocking that occurs with intensification of a nerve lesion.
A modification of the method which attempts to give a better assessment of signal blocking at a lesion is called "evoked electromyography". Evoked electromyography is most commonly utilized for evaluating facial nerve function (many physicians call the test "electroneurography" when used in this application). With this technique recording electrodes are placed on the skin overlying some of the muscle fibers that are controlled by the nerve rather than on the skin overlying the nerve. Following activation of the nerve by stimulation, these muscle fibers also become electrically active. The compound voltage from muscle fibers or "electromyographic potential" can be detected by the recording electrodes, amplified, and displayed on an oscilloscope. The physician can determine if the recorded response is abnormal in either latency (time delay before response) or in amplitude, indicating nerve signal slowing or blocking, respectively. Unfortunately, this technique also has severe limitations with respect to sensitivity and accuracy. Although the potential recorded from muscle is larger than that recorded from nerve in a normal person, the electromyographic potential recorded from most patients with nerve signal blocks is too small to be accurately measured. The recorded data must first be fed into a signal averager in order to obtain sufficient signal to noise ratio to measure the response. Very few physicians employ this test because of the complexity and cost required in using a signal averager. The test is also inaccurate. Slight changes in the electrode position produce changes in response amplitude or error that are greater than 15% and as large as 100%. The method has another limitation in that only those nerve fibers controlling muscle fibers in the vicinity of the recording electrodes can be assessed for damage. Nerve fibers controlling muscle fibers outside this region may also become damaged by a lesion, but no change in the recorded potential will be observed. Thus, the method represents a sampling technique and can not assess the status of the entire nerve.
The primary objects of the present invention are to provide a method and a device for evaluating the functions of regions of the nervous system, particularly peripheral nerves, that control muscular movements, using a method and device which is simpler, more sensitive, more accurate, less costly, and safer than prior methods and devices.
Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying drawings.